Department of Neurology, University of California Los Angeles, Los Angeles, CA, USA.
Department of Radiological Sciences, University of California Los Angeles, Los Angeles, CA, USA.
Med Phys. 2018 Jan;45(1):101-113. doi: 10.1002/mp.12640. Epub 2017 Nov 17.
CT Perfusion (CTP) is a widely used clinical imaging modality. However, CTP typically involves the use of substantial radiation dose (CTDI ≥~200 mGy). The purpose of this study is to present a low-dose CTP technique using a projection view-sharing reconstruction algorithm originally developed for dynamic MRI - "K-space Weighted Image Contrast" (KWIC).
The KWIC reconstruction is based on an angle-bisection scheme. In KWIC, a Fourier transform was performed along each projection to form a "k-space"-like CT data space, based on the central-slice theorem. As a projection view-sharing technique, KWIC preserves the spatiotemporal resolution of undersampled CTP data by progressively increasing the number of projection views shared for more distant regions of "k-space". KWIC reconstruction was evaluated on a digital FORBILD head phantom with numerically simulated time-varying objects. The numerically simulated scans were undersampled using the angle-bisection scheme to achieve 50%, 25%, and 12.5% of the original dose (288, 144, and 72 projections, respectively). The area-under-the-curve (AUC), time-to-peak (TTP), and full width half maximum (FWHM) were measured in KWIC recons and compared to fully sampled filtered back projection (FBP) reconstructions. KWIC reconstruction and dose reduction was also implemented for three clinical CTP cases (45 s, 1156 projections per turn, 1 s/turn, CTDI 217 mGy). Quantitative perfusion metrics were computed and compared between KWIC reconstructed CTP data and those of standard FBP reconstruction.
The AUC, TTP, and FWHM in the numerical simzulations were unaffected by the undersampling/dose reduction (down to 12.5% dose) with KWIC reconstruction compared to the fully sampled FBP reconstruction. The normalized root-mean-square-error (NRMSE) of the AUC in the FORBILD head phantom is 0.04, 0.05, and 0.07 for 50%, 25%, and 12.5% KWIC, respectively, as compared to FBP reconstruction. The cerebral blood flow (CBF) and cerebral blood volume had no significant difference between FBP and 50%, 25%, and 12.5% KWIC reconstructions (P > 0.05).
This study demonstrates that KWIC preserves perfusion metrics for CTP with substantially reduced dose. Clinical implementation will require further investigation into methods of rapid switching of a CT x-ray source.
CT 灌注（CTP）是一种广泛应用的临床成像方式。然而，CTP 通常需要使用大量的辐射剂量（CTDI≥~200mGy）。本研究旨在介绍一种低剂量 CTP 技术，该技术使用了一种最初为动态 MRI 开发的投影视图共享重建算法——“K 空间加权图像对比度”（KWIC）。
KWIC 重建基于角度二分法。在 KWIC 中，沿着每个投影进行傅里叶变换，形成基于中心切片定理的“k 空间”样 CT 数据空间。作为一种投影视图共享技术，KWIC 通过逐步增加用于更远的“k 空间”区域的投影视图共享数量来保留欠采样 CTP 数据的时空分辨率。KWIC 重建在具有数值模拟时变物体的数字 FORBILD 头部体模上进行了评估。使用角度二分法对数值模拟扫描进行欠采样，以实现原始剂量的 50%、25%和 12.5%（分别为 288、144 和 72 个投影）。在 KWIC 重建中测量曲线下面积（AUC）、峰值时间（TTP）和半峰全宽（FWHM），并与完全采样滤波反投影（FBP）重建进行比较。还对三个临床 CTP 病例（45 秒，每个旋转 1156 个投影，旋转速度为 1 秒/转，CTDI 为 217mGy）进行了 KWIC 重建和剂量降低。在 KWIC 重建的 CTP 数据和标准 FBP 重建的 CTP 数据之间计算并比较了定量灌注指标。
与完全采样的 FBP 重建相比，KWIC 重建在数值模拟中在达到 12.5%剂量的情况下不影响 AUC、TTP 和 FWHM。FORBILD 头部体模的 AUC 的归一化均方根误差（NRMSE）分别为 0.04、0.05 和 0.07，用于 50%、25%和 12.5%的 KWIC，而 FBP 重建的 AUC 的 NRMSE 为 0.04。在 FBP 和 50%、25%和 12.5%的 KWIC 重建之间，脑血流量（CBF）和脑血容量没有显著差异（P>0.05）。
本研究表明，KWIC 可以在大幅降低剂量的情况下保留 CTP 的灌注指标。临床实施将需要进一步研究 CT 射线源的快速切换方法。
